You create the sensation in the subject’s mind that he is threatened by more foes than he actually faces. Though the subject doesn’t actually perceive any additional enemies (and thus doesn’t waste any attacks on the phantasm), a creature affected by this spell is considered flanked, even if not threatened by other creatures. No amount of convincing by others can help the victim of this spell avoid its effect—only a successful saving throw against the spell when initially cast can help the target.